Variations in how much fluid is present in a person's thorax can take various forms and can have different causes. As an example, eating salty foods can result in retaining excessive fluid in the thorax, which is commonly referred to as “thoracic fluid,” and elsewhere. Posture changes can also affect the amount of thoracic fluid. For instance, moving from supine to standing can shift intravascular fluid away from the thorax toward the lower extremities.
Another cause of fluid build-up in a person's thorax is pulmonary edema, which involves buildup of extravascular fluid in the lungs. In pulmonary edema, fluid accumulates in extracellular spaces, such as the spaces between lung tissue cells. One cause of pulmonary edema is congestive heart failure (CHF), which is also sometimes referred to as “chronic heart failure,” or simply as “heart failure.” CHF can be conceptualized as an enlarged weakened heart muscle. The impaired heart muscle results in poor cardiac output of blood. As a result of such poor blood circulation, blood tends to pool in blood vessels in the lungs and becomes a barrier to normal oxygen exchange. This intravascular fluid buildup, in turn, results in the extravascular fluid buildup mentioned above. Accordingly, pulmonary edema can be an indicative and important condition associated with CHF.
Pulmonary edema, if it exists, may present a medical emergency that requires immediate care. Although it can sometimes prove fatal, the outlook for people possessing pulmonary edema can be good upon early detection and prompt treatment of the same. If left untreated, pulmonary edema can lead to death.
Implantable medical devices (IMD) include, among other things, cardiac rhythm management (CRM) devices such as pacers, cardioverters, defibrillators, cardiac resynchronization therapy (CRT) or coordination devices, and drug delivery systems. Such devices are often used for providing a diagnosis, a therapy, or both a diagnosis and a therapy.
An IMD's detection scheme is typically characterized by its “sensitivity” and “specificity.” Sensitivity generally refers to the ability of the detection scheme to effectively detect that which the caregiver desires the IMD to detect or treat. Sensitivity can be expressed as follows:Sensitivity=True Positives/(True Positives+False Negatives)  (Eq. 1)Specificity generally refers to the ability of the detection scheme to avoid improperly treating that which the caregiver determines that the device should not treat. Specificity can be expressed as follows:Specificity=True Negatives/(True Negatives+False Positives)  (Eq. 2)Ideally, an IMD would have both 100% sensitivity and 100% specificity. However, it is known in the art that for practical IMDs, there exists a tradeoff between sensitivity and specificity, such that no practical detection scheme can obtain the ideal.
Pulmonary edema is common in heart failure patients and, as discussed above, may be life threatening in many situations. Therefore, it is desirable to timely detect and treat pulmonary edema. Because of the potential severe (indeed life-threatening) consequences of failing to detect the presence of, and subsequently treat, pulmonary edema, it may be important that an IMD is configured to maximize sensitivity. At the same time, it may be important for such a device to possess a high level of specificity to avoid erroneous alerts and unneeded (and possibly harmful) treatment.